1. Field of the Invention
This invention relates to an electrical power generating system including an alternator producing polyphase alternating current at a high frequency, and, more particularly, to a rectifier and an auxiliary alternating current power supply within such a power generating system.
2. Summary of the Background Art
A number of electrical generating systems employ a polyphase alternator driven at a high speed by a relatively small gas turbine, called microturbine, to produce polyphase alternating current at a high frequency, such as 2.3 kHz. This high-frequency alternating current is then rectified, producing direct current that can be used for other purposes, such as generating 50- or 60-Hz alternating current. Individual generating systems of this kind are used to produce a few kilowatts to a few megawatts of electrical power. An alternator of this type generates electricity through the use magnetic fields established with permanent magnets.
One problem that has been identified with this type of generating system arises from the substantial internal inductive reactance of the alternator, which causes the output voltage of the alternator at its rated load to be significantly lower than its unloaded output voltage. As described in detail in U.S. Pat. App. Pub. No. 2003/0222459 A1, the disclosure of which is hereby included by reference, such an alternator may be a rotating drum-type machine having an annular air-gap, across which radial magnetic flux is established by a heteropolar permanent-magnet rotor. The inductive reactance of such an alternator is further described as restricting the available output power from the alternator by reducing the rated terminal voltage at rated current and by imposing a limit on the permissible current. The reduction in the rated terminal voltage at the rated current, which reduces both power output and efficiency, is present in any machine that includes significant internal inductive reactance. The limit imposed on the permissible current, which is accepted to restrict the internal reactance to an acceptable level, comes into play in an alternator design in which the rated current is limited by the need to hold the reactive voltage drop between an unloaded operating condition and operation at the rated load to an acceptable level, instead of by a need to limit heating within the alternator.
U.S. Pat. App. Pub. No. 2003/0222459 A1 further describes a polyphase alternator of this kind having a capacitor connected in series with each of the output terminals of a polyphase alternator of this kind, with the value of the capacitance of each capacitor being selected so that a drop in voltage from no-load to full-load occurring at the output terminal of the alternator is essentially offset at an output terminal at each of the capacitors.
This arrangement has been conventionally used to provide auxiliary power through the attachment of a transformer to the outputs of the capacitors, which are in turn attached to the output terminals of the alternator, along with the attachment of a rectifier to the outputs of the capacitors to provide output power. However, the addition of an inductive load, such as the windings of a polyphase transformer to provide power for auxiliary functions needed to run the microturbine, to the output terminals of such capacitors wired in series with a polyphase alternator has proven to increase the current flowing through the capacitors and out windings of the alternator in a way that places an undesirable limitation on the power that can be produced by the alternator. What is needed is a way to achieve the advantages of adding capacitance to the output lines of a high-frequency polyphase alternator when the output power from the alternator is used both to produce rectified direct current and to produce auxiliary power for use within the microturbine through a polyphase transformer, without significantly limiting the power that can be produced due to increased current associated with the reactance of the transformer windings.